1，Semaphore

适用于：限制未处理的特定资源请求（线程/操作）数量。

public class SemApp {public static void main(String[] args) {Runnable limitedCall = new Runnable() {final Random rand = new Random();final Semaphore available = new Semaphore(3);int count = 0;public void run() {int time = rand.nextInt(15);int num = count++;try {available.acquire();System.out.println("Executing " + "long-running action for " + time + " seconds... #" + num);Thread.sleep(time * 1000);System.out.println("Done with #" + num + "!");available.release();} catch (InterruptedException intEx) {intEx.printStackTrace();}}};for (int i=0; i<10; i++)new Thread(limitedCall).start();}
}

程序中实现了，限制了同时打开的任务数是三个。

结果写道

Executing long-running action for 9 seconds... #0
Executing long-running action for 7 seconds... #2
Executing long-running action for 0 seconds... #1
Done with #1!
Executing long-running action for 1 seconds... #3
Done with #3!
Executing long-running action for 6 seconds... #4
Done with #2!
Executing long-running action for 5 seconds... #5
Done with #4!
Executing long-running action for 6 seconds... #6
Done with #0!
Executing long-running action for 12 seconds... #7
Done with #5!
Executing long-running action for 1 seconds... #8
Done with #8!
Executing long-running action for 12 seconds... #9
Done with #6!
Done with #7!
Done with #9!

对于特定资源和线程访问数的限定Semaphore是个不错的选择。这里使用了Semaphore一次使用和释放一个通行证，Semaphore还可以一次使用和释放多个通行证。

2，CountDownLatch

public class CDLApp {public static void main(String[] args)throws InterruptedException, java.io.IOException {System.out.println("Prepping...");Race r = new Race("horse 1", "horse 2",    "horse 3",    "horse 4","horse 5",    "horse 6",    "horse 7",    "horse 8");System.out.println("It's a race of " + r.getDistance() + " lengths");System.out.println("Press Enter to run the race....");System.in.read();r.run();}
}
class Race {private Random rand = new Random();private int distance = rand.nextInt(250);private List<String> horses = new ArrayList<String>();public Race(String... names) {this.horses.addAll(Arrays.asList(names));}public int getDistance() {return distance;}public void run() throws InterruptedException {System.out.println("And the horses are stepping up to the gate...");final CountDownLatch start = new CountDownLatch(1);final CountDownLatch finish = new CountDownLatch(horses.size());final List<String> places =Collections.synchronizedList(new ArrayList<String>());for (final String h : horses) {new Thread(new Runnable() {public void run() {try    {start.await();//等待赛马开始System.out.println(h + " stepping up to the gate...");int traveled = 0;while (traveled < distance)    {//前进一次 暂停 0-2 秒Thread.sleep(rand.nextInt(3) * 1000);//前进 0-14 米traveled += rand.nextInt(15);System.out.println(h + " advanced to " + traveled + "!");}System.out.println(h + " crossed the finish!");places.add(h);finish.countDown();//一个马到达终点就 减去一， 倒数完成比赛结束}catch (InterruptedException intEx) {System.out.println("ABORTING RACE!!!");intEx.printStackTrace();}}}).start();}System.out.println("And... they're off!");start.countDown();//开始赛马finish.await();//等待所有的马跑完System.out.println("============================================");System.out.println("And we have our winners!");System.out.println("<"+places.get(0)+">" + " took the gold...");System.out.println("<"+places.get(1)+">" + " got the silver...");System.out.println("and " + "<"+places.get(2)+">" + " took home the bronze.");System.out.println("============================================");}
}

上面的程序利用CountDownLatch 描述一个赛马的过程，首先让所有马在门闩那里（start.countDown();//开始赛马）等待开始，每匹马冲过终点后（finish.countDown），finish.await()等待所有的马冲过终点后，发奖牌。

3，ScheduledExecutorService

public class Ping {public static void main(String[] args) {ScheduledExecutorService ses =Executors.newScheduledThreadPool(1);Runnable pinger = new Runnable() {public void run() {System.out.println("PING!");}};ses.scheduleAtFixedRate(pinger, 1, 2, TimeUnit.SECONDS);}
}

模拟心脏每隔2秒钟跳动一次。顺便说一下，如果用户希望取消心跳，scheduleAtFixedRate 调用将返回一个 ScheduledFuture 实例，它不仅封装了结果（如果有），还拥有一个 cancel 方法来关闭计划的操作。

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java多线程 Semaphore CountDownLatch ScheduledExecutorService

1，Semaphore

2，CountDownLatch

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